JP2017198920A - Image formation apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image formation apparatus capable of stabilizing the quantity of heat to be imparted to a heat conduction member without any excessive noise to an external power supply system while letting a halogen heater maintain a halogen cycle.SOLUTION: A second halogen heater (332) is lower in rated electric power than a first halogen heater (331). A PWM control part (30C) monitors an actual temperature of the heat transfer member (31), selects an object of application with a pulse voltage according to a difference between the actual temperature and a target temperature, and changes the duty ratio of the pulse voltage with temporal change in the difference. A switching conversion part (30S) applies the pulse voltage to the object of application at a duty ratio. The PWM control part when having selected the first halogen heater changes the object of application with the pulse voltage from the first halogen heater to the second halogen heater if the duty ratio having been changed decreases below a lower limit of the duty ratio satisfying such a condition that the quantity of heat thereof is large enough to maintain a halogen cycle.SELECTED DRAWING: Figure 7

Description

  The present invention relates to an electrophotographic image forming apparatus, and more particularly to temperature control of a fixing unit thereof.

  An electrophotographic image forming apparatus such as a laser printer, a facsimile machine, or a copier includes a fixing unit. The fixing unit fixes the toner image on the sheet by welding the toner to the sheet by heating and pressing. The fixing unit includes a member that contacts the sheet and transfers heat, and a member that heats the heat transfer member. Rotating bodies such as rollers and belts are mainly used as the heat transfer member, and halogen heaters are mainly used as the heating member.

  As a control for the halogen heater, for example, one disclosed in Patent Document 1 is known. This control is broadly divided into a temperature rise control and a temperature control. “Temperature increase control” refers to control for continuously heating the heat transfer member to the heater during warm-up or recovery. In the temperature rise control for the halogen heater, the voltage applied to the heater is continuously turned on to continuously light the heater. As a result, the amount of heat supplied to the heat transfer member is maintained at the maximum value, so that the temperature of the heat transfer member quickly rises to the target value. “Temperature control” refers to control for adjusting the amount of heat generated by the heater so that the temperature of the heat transfer member is maintained at a target value during printing or standby. The temperature control for the halogen heater makes the heater blink at a high frequency by converting the voltage applied to the heater into a pulse train with a constant frequency, and the duty ratio of the pulse voltage is changed minutely by pulse width modulation (PWM). Let Thus, the amount of heat supplied from the heater to the heat transfer member is adjusted with high accuracy, so that the temperature of the heat transfer member is maintained near the target value. By switching between the temperature increase control and the temperature control, both the temperature increase and the heat retention of the heat transfer member are realized with one halogen heater.

With recent advances in image forming apparatuses that are faster, more functional, more reliable, and more energy efficient, the fuser has a wider range of heat applied to the sheet, and the temperature of the heat transfer member is increased. Further, it is required to control with high accuracy. One way to meet this requirement is to use multiple halogen heaters.
For example, Patent Document 2 discloses suppression of inrush current by this pluralization. The higher the rated power of the halogen heater, the faster the temperature rises. However, the increase in rated power increases the inrush current that occurs at the start of energization of the heater. An excessive inrush current has a risk of temporarily dropping the external power supply voltage. In the image forming apparatus disclosed in Patent Document 2, two heaters having different resistance values are mounted on the fixing unit, and current is supplied to only one of them when starting energization. Thereby, inrush current is suppressed. Thereafter, when the temperature of the heating roller rises to a predetermined value, current is passed in parallel to both heaters to accelerate the temperature rise.

Patent Document 3 discloses a technique in which a central portion and an end portion in the lateral direction of the fixing belt are separately heated with different heaters. The heating range of the belt is optimized according to the sheet size. This reduces the power consumption of the heater.
In the image forming apparatus disclosed in Patent Document 4, two halogen heaters having different rated powers are mounted on the fixing unit, and the ratio of the lighting time between the heaters is changed in accordance with the sheet conveyance speed, so that Optimize the amount of heating. By using a plurality of halogen heaters, the range of the heating amount is expanded and the heating amount is set finely, so that the heating efficiency is improved.

JP 2009-069371 A JP-A-5-324101 JP 2014-178370 A JP 2012-181348 A JP 2002-063981 A

  In order to shorten the warm-up time and the recovery time, it is preferable to set the rated power of the halogen heater high to increase the temperature of the heat transfer member. On the other hand, in order to reduce power consumption during the standby period, it is desirable to change the amount of heat generated by the heater more minutely. In order to realize these with a single heater, the lower limit of the heat generation amount of the heater in the temperature control, that is, the lower limit of the duty ratio of the pulse voltage must be further lowered. However, a decrease in the amount of heat generated with this decrease in the duty ratio causes the tube wall temperature of the heater to exceed the lower limit of the temperature at which the halogen cycle is sustainable, approximately 250 ° C. (hereinafter simply referred to as “the lower limit of the tube wall temperature”). Therefore, it is difficult to further extend the life of the heater.

  As a device for reducing the amount of heat given to the heat transfer member by the heater while maintaining the tube wall temperature of the halogen heater at the lower limit or higher during the standby period, for example, the control disclosed in Patent Document 1 is known. This control alternates between a period in which the halogen heater blinks at a duty ratio that can maintain the tube wall temperature above the lower limit during the standby period, and a period in which the heater is turned off and continuously turned off. Set to. In the blinking period, the tube wall temperature is maintained above the lower limit, so that the halogen cycle continues. On the other hand, since the extinguishing period is sandwiched between the blinking periods, the amount of heat to the heat transfer member can be suppressed by the ratio of the time lengths of both periods.

  However, in this control, the amount of current of the heater is periodically increased or decreased by alternately bringing the blinking period and the extinguishing period. This increase / decrease is transmitted as noise to an external power supply system such as a commercial power supply, and there is a risk of causing flicker in a fluorescent lamp included in the illumination connected to the power supply system. Further, since the temperature of the heat transfer member greatly decreases every time the lamp is turned off, the temperature ripple increases, and it is difficult to increase the accuracy of temperature control and further reduce power consumption.

  The object of the present invention is to solve the above-mentioned problems, and in particular, the amount of heat given to the heat transfer member is further reduced without giving excessive noise to the external power supply system while maintaining the halogen cycle in the halogen heater. An object of the present invention is to provide an image forming apparatus which can be stabilized to a value.

  An image forming apparatus according to one aspect of the present invention includes an image forming unit that forms a toner image on a sheet, and a fixing unit that thermally fixes the toner image on the sheet. The fixing unit includes a heat transfer member that contacts the sheet and transfers heat, a first halogen heater that heats the heat transfer member, and a second halogen that has a lower rated power than the first halogen heater and heats the heat transfer member. The actual temperature of the heater and the heat transfer member is monitored, and one or both of the first halogen heater and the second halogen heater is selected as the pulse voltage application target according to the difference between the actual temperature and the target temperature. The pulse width modulation (PWM) control unit that changes the duty ratio of the pulse voltage with time according to the change over time of the difference, the pulse control unit that applies the pulse voltage to the pulse voltage application target selected by the PWM control unit, And a switching converter that applies the changed duty ratio. The PWM control unit sets the lower limit of the duty ratio of the pulse voltage that satisfies the condition that the heat generation amount of the first halogen heater can maintain the halogen cycle when the first halogen heater is selected as the pulse voltage application target. Then, it is confirmed whether or not the duty ratio after the change is below. If the duty ratio after the change is below the lower limit, the pulse voltage application target is switched from the first halogen heater to the second halogen heater.

The PWM control unit may check whether or not the changed duty ratio falls below the lower limit during a standby period between the image forming unit and the fixing unit.
When the first halogen heater is selected as the pulse voltage application target, the PWM control unit sets the first upper limit of the duty ratio of the pulse voltage that satisfies the condition that the switching converter can operate in the current discontinuous mode. The second halogen heater may be added to the application target of the pulse voltage if it is confirmed whether or not the changed duty ratio exceeds the first upper limit. The PWM control unit confirms whether the changed duty ratio exceeds the first upper limit during at least one of the warm-up period, the recovery period, and the print period between the image forming unit and the fixing unit. May be.

  The PWM control unit sets the second upper limit of the duty ratio of the pulse voltage that satisfies the condition that the switching converter can operate in the current discontinuous mode when the second halogen heater is selected as the pulse voltage application target. Whether or not the changed duty ratio is exceeded is confirmed. If the changed duty ratio exceeds the second upper limit, the pulse voltage application target may be switched from the second halogen heater to the first halogen heater. The PWM control unit checks whether the changed duty ratio exceeds the second upper limit during at least one of the warm-up period, the recovery period, and the print period between the image forming unit and the fixing unit. May be.

  The switching converter includes a drive circuit that generates a pulse signal with a duty ratio changed by the PWM controller and a switching element that is turned on / off according to the pulse signal, and outputs a pulse voltage with the duty ratio changed by the PWM controller. You may have a step-down chopper and the switch part which switches the output destination of the pulse voltage from the step-down chopper to the pulse voltage application object which the PWM control part selected. The PWM control unit may cause the switch unit to switch the output destination of the pulse voltage from the step-down chopper while the switching element of the step-down chopper is turned off. The switch unit may include an electromagnetic relay that opens and closes a contact between the step-down chopper and the first halogen heater according to a signal from the PWM control unit, and opens and closes a contact between the step-down chopper and the second halogen heater. Good.

  In the image forming apparatus according to the present invention, when the PWM control unit selects the first halogen heater as the pulse voltage application target, the pulse satisfies the condition that the heat generation amount of the first halogen heater can maintain the halogen cycle. It is confirmed whether or not the duty ratio after the change is lower than the lower limit of the duty ratio of the voltage. If the duty ratio after the change is lower, the pulse voltage application target is switched from the first halogen heater to the second halogen heater. Thus, the image forming apparatus can stabilize the amount of heat applied to the heat transfer member to a smaller value without causing excessive noise to the external power supply system while maintaining the halogen cycle in the halogen heater.

FIG. 1A is a perspective view illustrating an appearance of an image forming apparatus according to an embodiment of the present invention. (B) is a schematic cross-sectional view of the image forming apparatus along a line bb shown in (a). FIG. 2A is a schematic perspective view of a roller pair included in the fixing unit illustrated in FIG. 1B and their drive mechanism. (B) is sectional drawing of the roller pair along the straight line bb which (a) shows. (A) is a side view of the halogen heater shown in FIG. (B) is a schematic cross-sectional view of a glass tube showing a halogen cycle generated between halogen gas molecules and tungsten atoms in the internal space of the glass tube shown in (a). (C) is a graph which shows the relationship between the inner-wall temperature of the glass tube, and the lifetime of a halogen heater. FIG. 2 is a block diagram illustrating a configuration of an electronic control system of the image forming apparatus illustrated in FIG. 1. FIG. 5 is a circuit diagram of a switching converter shown in FIG. 4. It is a graph which shows the waveform of the electric current / voltage in each part of the switching conversion part shown in FIG. (A) shows the waveform of the input voltage VI of the rectifier, (b) shows the waveform of the output voltage VR of the rectifier, and (c) shows the pulse signal applied by the IGBT drive circuit to the gate of the switching element. The waveform of VP is shown. (D) shows the waveform of the voltage VD across the regenerative diode when the step-down chopper is connected to at least one of the halogen heaters, and (e) shows the waveform of the output current IO of the step-down chopper in the same case. . (F) is a graph showing waveforms of the pulse signal VP, the emitter current IE, and the collector-emitter voltage VCE before and after the switching element is turned on. (A) is a graph which shows the output power range which can use the halogen heater shown in FIG. (B) is a table | surface which shows the control pattern with respect to the halogen heater by the switching conversion part shown in FIG. (A) is a graph which shows the time-dependent change of the temperature of the heating roller shown in FIG. 2 after the starting time t0 of the image forming apparatus shown in FIG. (B) is a correspondence table between the initial temperature T0 of the heating roller and the control pattern selected by the PWM controller shown in FIG. 5 according to the temperature. It is a flowchart of the temperature control of the heating roller shown in FIG. 2 by the fixing unit shown in FIG. It is a flowchart of control in standby mode among temperature control by step S103 which FIG. 9 shows. It is a flowchart of PWM control with respect to the halogen heater shown in FIG.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[Appearance of image forming apparatus]
FIG. 1A is a perspective view showing an external appearance of an image forming apparatus 100 according to an embodiment of the present invention. The image forming apparatus 100 is an electrophotographic color printer, that is, a color laser printer. A paper discharge tray 41 is provided on the upper surface of the printer 100 casing, and accommodates sheets discharged from a paper discharge port 42 opened in the back thereof. An operation panel 51 is attached in front of the paper discharge tray 41. On the operation panel 51, a display with a built-in touch panel is arranged in addition to various mechanical push buttons. The display displays a graphics user interface (GUI) screen such as an operation screen and an input screen for various information. The touch panel accepts user input operations through gadgets including GUI screens such as icons, virtual buttons, menus, and toolbars. A sheet feeding cassette 11 is removably attached to the bottom of the printer 100, and a bundle of sheets is accommodated therein. “Sheet” refers to a thin film or thin plate material, article, or printed material made of paper or resin. The type of sheet that can be stored in the paper feed cassette 11, that is, the paper type, is plain paper, high-quality paper, color paper, or coated paper, and the size is A3, A4, A5, or B4. Furthermore, the posture of the sheet can be set to either vertical or horizontal.

[Internal structure of image forming apparatus]
FIG. 1B is a schematic cross-sectional view of the printer 100 along the line bb shown in FIG. As shown in the figure, the printer 100 includes a feeding unit 10, an image forming unit 20, a fixing unit 30, and a paper discharge unit 40.
The feeding unit 10 feeds the sheet SH1 from the paper feeding cassette 11 to the image forming unit 20 one by one using the paper feeding roller 12.

  The image forming unit 20 is a tandem type, and forms a color or monochrome toner image on the sheet SH2 sent from the feeding unit 10. Specifically, each of the four photoconductor (PC) units 20Y, 20M, 20C, and 20K first charges the surface of the photoconductor (PC) drums 21Y, 21M, 21C, and 21K, and the charged portion exposes the exposure unit 22. Shed light from Since these lights are modulated according to the gradation value distribution of yellow (Y), magenta (M), cyan (C), and black (K) indicated by the image data, each surface of the PC drum 21Y,. An electrostatic latent image of each color is formed. Next, the PC units 20Y,... Develop the electrostatic latent image with Y, M, C, and K toners. Thereafter, the four-color toner images are sequentially transferred from the surface of the PC drum 21Y,... To the surface of the intermediate transfer belt 24 by the electric field between the primary transfer rollers 23Y, 23M, 23C, 23K and the PC drum 21Y,. Transferred to the same position. Thus, one color toner image is formed at that position. Thereafter, when this color toner image passes through the nip between the drive pulley 24R of the intermediate transfer belt 24 and the secondary transfer roller 25, the sheet is simultaneously fed to the same nip by the electric field between the both 24R and 25. Transferred to the surface of SH2. The sheet SH2 is peeled off from the secondary transfer roller 25 and then sent out to the fixing unit 30.

  The fixing unit 30 thermally fixes the toner image on the sheet SH <b> 3 sent out from the image forming unit 20. Specifically, when the sheet SH3 is passed through the nip between the heating roller 31 and the pressure roller 32, the heating roller 31 applies heat of a built-in heater to the surface of the sheet SH3, and the pressure roller No. 32 applies pressure to the heated portion of the sheet SH3 and presses it against the heating roller 31. The toner is welded onto the surface of the sheet SH3 by the heat from the heating roller 31 and the pressure from the pressure roller 32. Thereafter, the heating roller 31 and the pressure roller 32 send out the sheet SH3 from the fixing unit 30.

The paper discharge unit 40 conveys the sheet SH3 sent from the fixing unit 30 to the paper discharge tray 41. Specifically, when the leading end of the sheet SH3 reaches the nip between the fixing unit 30 and the discharge roller 43, the discharge roller 43 pulls the leading end into the discharge port 42. As a result, the sheets SH3 sequentially pass through the discharge outlet 42 from the leading edge and are stacked on the discharge tray 41.
[Fixing part structure]
2A is a schematic perspective view of the roller pairs 31 and 32 included in the fixing unit 30 and their drive mechanisms, and FIG. 2B is along a straight line bb shown in FIG. It is sectional drawing of the roller pair 31 and 32. FIG. As shown in these drawings, the fixing unit 30 includes a heating roller 31, a pressure roller 32, two halogen heaters 331 and 332, a temperature sensor 35, a motor 36, and a torque transmission mechanism 37.

The heating roller 31 and the pressure roller 32 have rotational axes arranged in parallel with each other, and their outer peripheral surfaces are in contact with each other. The sheet SH2 fed from the image forming unit 20 is sandwiched between the contact portions, that is, the nip.
The heating roller 31 includes a cored bar 311, an elastic body layer 312, and a release layer 313. The cored bar 311 is a cylindrical member having a diameter of several tens mm, for example, and is mainly made of a metal such as aluminum or iron. The elastic body layer 312 is a layer mainly made of a highly elastic heat-resistant resin such as silicone rubber and covering the outside of the core metal 311 and has a thickness of, for example, less than 1 mm. The release layer 313 is a thin film such as a fluororesin that covers the outer side of the elastic body layer 312, and forms the outer peripheral surface of the heating roller 31. The release layer 313 prevents a phenomenon (offset phenomenon) in which the toner melted by the heating by the heating roller 31 is transferred from the surface of the sheet SH2 to the outer peripheral surface of the heating roller 31.

  The pressure roller 32 includes a cored bar 321, an elastic body layer 322, and a release layer 323. The cored bar 321 is a cylindrical member having a diameter of several tens mm, for example, and is mainly made of a metal such as aluminum or iron. The elastic body layer 322 is a layer mainly made of a highly elastic heat-resistant resin such as silicone rubber that covers the outside of the cored bar 321. The thickness of this layer is several mm, for example, and is larger than the thickness of the elastic body layer 312 of the heating roller 31. The release layer 323 is a thin film such as a fluororesin that covers the outside of the elastic body layer 322, and forms the outer peripheral surface of the pressure roller 32. The pressure roller 32 receives a force toward the heating roller 31 from a biasing member such as a spring or an electromagnet not shown in FIG. As a result, the portion of the pressure roller 32 positioned at the nip is pressed against the heating roller 31, and as shown in FIG. 2B, the portion is deformed so as to be recessed. Due to this pressure and deformation of the pressure roller 32, a sufficient amount of heat is transmitted from the heating roller 31 to the portion of the sheet SH2 sandwiched between the nips, so that the toner adhering to the portion of the sheet SH2 does not leave unevenness. Fix on the surface.

  The two halogen heaters 331 and 332 are installed in the internal space of the core metal 311 of the heating roller 31. Each of the heaters 331 and 332 is a thin rod-shaped halogen lamp that extends in the longitudinal direction in the inner space of the cored bar 311 and heats the entire cored bar 311 from the inside by heat radiation accompanying light emission. Since this heat is transmitted to the outer peripheral surface through the elastic body layer 312 and the release layer 313 of the heating roller 31, the temperature is maintained in the range of hundreds of degrees Celsius to several hundred degrees Celsius, for example. Due to this high temperature, the toner adhering to the surface of the sheet SH2 is melted.

  The temperature sensor 35 is a non-contact type temperature sensor using a thermopile. The thermopile absorbs the heat radiated from the surface to be measured by the black light receiving surface, and detects the temperature rise of the light receiving surface due to the absorption by connecting a number of thermocouples in series. The temperature sensor 35 is opposed to the outer peripheral surface of the heating roller 31 at a predetermined distance, and the surface temperature of the heating roller 31 (hereinafter referred to as “the temperature of the heating roller 31” is abbreviated from the output of the thermopile accompanying heat radiation from that surface) )). The measured value is used to control the amount of heat generated by the halogen heaters 331 and 332.

  The motor 36 is, for example, a direct current brushless (BLDC) motor, and its shaft is connected to one end of a torque transmission mechanism 37. The other end of the transmission mechanism 37 is connected to one end of a core bar 311 of the heating roller 31. The transmission mechanism 37 includes, for example, a plurality of gears. The gear maintains the rotation speed of the heating roller 31 at a predetermined ratio with respect to the rotation speed of the shaft of the motor 36. The motor 36 transmits the torque to the heating roller 31 through the transmission mechanism 37 and rotates it as a driving roller. Further, the frictional force generated in the nip with this rotation rotates the pressure roller 32 as a driven roller.

[Halogen heater]
-Structure-
FIG. 3A is a side view of the first halogen heater 331. The structure shown in this figure is the same for the second halogen heater 332. As shown in the figure, the first halogen heater 331 includes a glass tube 33A, a filament 33B, a sealed gas 33C, a sealing portion 33D, and a base 33E. The glass tube 33A is an elongated circular tube made of, for example, quartz glass, and can withstand a high temperature of several hundred degrees Celsius due to the high heat radiated from the heater 331. The filament 33B is, for example, a coiled tungsten wire, and extends in the longitudinal direction in the internal space of the glass tube 33A. The sealed gas 33C is a mixed gas of an inert gas and a trace amount of halogen gas. For example, the inert gas is nitrogen, argon, or krypton, and the halogen gas is iodine, bromine, chlorine, or a compound thereof. The halogen gas returns the tungsten atoms to the filament 33B by repeating a cyclic chain reaction (halogen cycle) with the tungsten atoms vaporized by evaporation from the filament 33B. The presence of the halogen cycle makes the filament of the halogen lamp have a longer life than that of the incandescent bulb. The sealing portion 33D is each end portion in the longitudinal direction of the glass tube 33A and is hermetically sealed. Molybdenum foil 33F is embedded in the sealing portion 33D, and one end thereof is exposed to the internal space of the glass tube 33A and connected to the filament 33B. The other end of the molybdenum foil 33F is electrically connected to the base 33E in the sealing portion 33D. The base 33E is made of, for example, ceramic or a metal having high heat resistance, and fixes both ends of the glass tube 33A and makes the filament 33B conductive to an external power source through the molybdenum foil 33F.

-Halogen cycle-
FIG. 3B is a schematic cross-sectional view of the glass tube 33A showing a halogen cycle generated between halogen gas molecules and tungsten atoms in the internal space of the glass tube 33A. As shown in this figure, while the heater 331 is energized, it is sandwiched between a filament 33B of several thousand degrees Celsius and a glass tube 33A that is cooled to several hundred degrees Celsius by the outside air. The space is divided into a high temperature region HTR near the filament 33B and a low temperature region LTR near the inner wall of the glass tube 33A. In the high temperature region HTR, in addition to the halogen molecules X, tungsten atoms W vaporized from the filament 33B fly around. When the tungsten atoms W move from the high temperature region HTR to the low temperature region LTR by diffusion, they are cooled by the surrounding gas and bonded to the two halogen molecules X to form tungsten halide molecules WX ₂ . Since tungsten halide is volatile, if the inner wall of the glass tube 33A is sufficiently hot, the tungsten halide molecule WX ₂ continues to float in the low temperature region LTR without adhering to the inner wall. When the tungsten halide molecule WX ₂ moves from the low temperature region LTR to the high temperature region HTR along with the convection of the sealed gas 33C, it is heated from the surrounding gas or filament 33B and dissociated into tungsten atoms W and halogen molecules X. The dissociated tungsten atoms W are deposited on the filament 33B.

  Thus, the halogen cycle acts to return the tungsten atoms vaporized from the filament 33B to the filament 33B again. Due to this action, the filament 33B is less likely to become thin regardless of the evaporation of tungsten, and thus has high resistance to disconnection. The halogen cycle also acts to prevent vaporized tungsten atoms from adhering to the inner wall of the glass tube 33A. By this action, the phenomenon that the inner wall of the glass tube 33A turns black with the solidification of the attached tungsten atoms hardly occurs (blackening), so that the transmittance of the glass tube 33A is maintained high for a long time. Therefore, the longer the duration of these actions by the halogen cycle, the longer the life of the halogen heater 33.

  FIG. 3C is a graph showing the relationship between the inner wall temperature of the glass tube 33 </ b> A and the lifetime of the halogen heater 331. The vertical axis of this graph represents a relative value with respect to the longest life of the heater 331. As this graph shows, if the inner wall temperature of the glass tube 33A is kept only below about 250 ° C., the life of the heater 331 is remarkably shortened. This is because the inner wall temperature of the glass tube 33A is too low for the vaporization of tungsten halide molecules, so that the blackening of the inner wall by the tungsten halide molecules proceeds and the halogen cycle becomes unsustainable. Therefore, in order to extend the life of the heater 331, it is important to maintain the inner wall temperature of the glass tube 33A at about 250 ° C. or higher and to maintain the halogen cycle.

[Electronic control system of image forming apparatus]
FIG. 4 is a block diagram illustrating the configuration of the electronic control system of the printer 100. In this control system, in addition to the elements 10, 20, 30, and 40 of the printer 100, the operation unit 50 and the main control unit 60 are connected to each other through a bus 90 so as to communicate with each other.
−Driver−
Each element 10,..., 40 of the printer 100 includes drive units 10D, 20D, 30D, 40D. Although not shown in FIG. 4, each drive unit 10 </ b> D includes actuators, control circuits, and drive circuits for movable members such as transport rollers 12, 21, 24 </ b> R, 25, 31, 32, 43, PC drums 21 </ b> Y,. Including a combination of The actuator is, for example, a BLDC motor. The control circuit is an electronic circuit such as a microprocessor (MPU / CPU), an application specific integrated circuit (ASIC), or a programmable integrated circuit (FPGA), and is an actual control amount fed back from an actuator, for example, a motor. If there is, the target value of the voltage applied to the actuator is instructed to the drive circuit based on the rotational speed. The drive circuit is a switching converter, and uses a power transistor such as a field effect transistor (FET) or an insulated gate bipolar transistor (IGBT) as a switching element to apply a voltage to the actuator. By using feedback control by these control circuits and drive circuits, each drive unit 10D,... Controls the control amount of the actuator to a target value instructed from the main control unit 60.

-Operation part-
The operation unit 50 is an entire interface between a user mounted on the printer 100 and an external electronic device, and accepts a job processing request and image data to be printed through a user operation or communication with the external electronic device. These are transmitted to the main controller 60. As shown in FIG. 4, the operation unit 50 includes an operation panel 51 and an external interface (I / F) 52. As shown in FIG. 1A, the operation panel 51 includes a push button, a touch panel, and a display. The operation panel 51 displays a GUI screen on this display. The operation panel 51 also identifies what is pressed by the user from among the push buttons, or detects the position touched by the user from the touch panel, and transmits information related to the identification or detection to the main control unit 60 as operation information. . In particular, when the print job input screen is displayed on the display, the operation panel 51 displays the printing conditions such as the size of the sheet to be printed, the paper type, the orientation (separately between the portrait and landscape orientation), the number of copies, and the image quality. Are received from the user, and items indicating these conditions are incorporated into the operation information. The external I / F 52 includes a USB port or a memory card slot, through which image data to be printed is taken directly from an external storage device such as a USB memory or a hard disk drive (HDD). The external I / F 52 further includes a communication port connected to the external network NTW by wire or wireless, and receives image data to be printed from another electronic device through the network NTW.

−Main control unit−
The main controller 60 is an integrated circuit mounted on a single printed circuit board installed inside the printer 100. As shown in FIG. 4, the main control unit 60 includes a CPU 61, a RAM 62, and a ROM 63. The CPU 61 is composed of an MPU and executes various firmware. The RAM 62 is a volatile semiconductor memory device such as a DRAM or an SRAM, and provides a work area when the CPU 61 executes firmware to the CPU 61 and stores image data to be printed received by the operation unit 50. The ROM 63 is configured by a combination of a non-writable nonvolatile storage device and a rewritable nonvolatile storage device. The former stores firmware, and the latter includes a semiconductor memory device such as an EEPROM, flash memory, and SSD, or an HDD, and provides a storage area for environment variables and the like to the CPU 61.

The main control unit 60 implements various functions as a control subject for the other elements 10, 20,... According to various firmware executed by the CPU 61. For example, the main control unit 60 displays a GUI screen on the operation unit 50 to accept a user input operation. Further, the main control unit 60 determines the operation mode of the printer 100 according to the operation information from the operation unit 50.
The operation mode of the printer 100 includes, for example, “operation”, “standby (low power)”, and “sleep”. “Operation mode” refers to an operation mode in which the printer 100 processes a print job. For example, the feeding unit 10 continuously feeds the number of sheets indicated by the operation information, the image forming unit 20 repeats the formation of the toner image and the transfer to the sheet, and the fixing unit 30 heats and heats the sheet. Continue with pressure. “Standby mode” refers to an operation mode in which the printer 100 stands by in a state where a job can be executed. Specifically, the feeding unit 10 and the image forming unit 20 are stopped, and the fixing unit 30 preheats the heating roller 31 with the halogen heaters 331 and 332 and keeps it at an appropriate temperature. “Sleep mode” refers to an operation mode in which the printer 100 minimizes power consumption. For example, the fixing unit 30 is stopped in addition to the feeding unit 10 and the image forming unit 20, and in particular, power supply to the halogen heaters 331 and 332 is shut off.

  The main control unit 60 responds to various events occurring in the printer 100, for example, completion of job processing, pressing of a stop button or a power button, detection of a gesture by a touch panel, reception of a job processing request or a stop command from the network. The operation mode of the printer 100 is switched. The main control unit 60 further provides information necessary for the switching to each element 10 of the printer 100. For example, when instructing the operation mode, the main control unit 60 designates the paper type and the number of sheets to be fed, the timing of rotation of each of the conveyance rollers 12,. The image forming unit 20 is provided with image data and image forming timing, and the fixing unit 30 is designated with a target temperature of the heating roller 31 or a heat generation amount of the halogen heaters 331 and 332.

-Fixing part-
The control system of the fixing unit 30 is an integrated circuit such as an MPU / CPU, ASIC, or FPGA mounted on a printed circuit board different from the main control unit 60. This control system includes a pulse width modulation (PWM) control unit 30C and a switching conversion unit 30S in addition to the drive unit 30D.
The PWM control unit 30C is an electronic circuit such as an MPU / CPU, an ASIC, an FPGA, etc., monitors the actual temperature of the heating roller 31, and according to the difference between the actual temperature and the target temperature, One or both of the second halogen heater 332 and the second halogen heater 332 are selected as application targets of a pulse voltage having a constant frequency (for example, 20 kHz, hereinafter referred to as “PWM frequency”). The PWM control unit 30C further changes the duty ratio of the pulse voltage over time in accordance with the change with time of the difference between the actual temperature of the heating roller 31 and the target temperature. Specifically, the PWM control unit 30C obtains the difference between the measured value of the temperature of the heating roller 31 fed back from the temperature sensor 35 and the target value of the temperature, and the duty of the pulse voltage so that the difference is narrowed. The ratio is changed, and the changed duty ratio is notified to the switching converter 30S.

  The switching conversion unit 30S receives AC power from an external power source such as a commercial AC power source, converts it to DC power, and supplies it to the halogen heaters 331 and 332. In particular, the switching converter 30S outputs a pulse voltage having a PWM frequency to a target to be applied selected by the PWM controller 30C using a power transistor such as an FET or IGBT as a switching element. The switching conversion unit 30S further adjusts the duty ratio of the pulse voltage to a value changed by the PWM control unit 30C by PWM.

[Switching converter]
-Circuit configuration-
FIG. 5 is a circuit diagram of the switching converter 30S. As shown in this figure, the switching conversion unit 30S includes a rectification unit 381, a noise filter 382, a step-down chopper 383, an IGBT drive circuit 384, and a switch unit 385.

The rectifying unit 381 is, for example, a full-wave rectifying circuit, and converts the AC voltage VI into a DC voltage VR by inverting the polarity of the negative part of the AC voltage VI applied from the external power supply OPS.
The noise filter 382 is, for example, a π-type three-terminal LC filter, and includes a reactor L1 and two capacitors C1 and C2. One end of the reactor L1 is connected to one of the first output terminals 1A of the rectifying unit 381. Each of the capacitors C1 and C2 is connected between a different end of the reactor L1 and the other output terminal of the rectifying unit 381. The noise filter 382 is a low-pass filter and mainly prevents high-frequency noise generated by the step-down chopper 383 from propagating toward the external power source OPS.

  The step-down chopper 383 is a switching converter that converts an input DC voltage into a lower DC voltage by repeatedly turning on and off the switching element, and includes a reactor L2, a regenerative diode D1, and a switching element SW. One end of the reactor L2 is connected to the first output terminal 1A of the rectifying unit 381 through the reactor L1 of the noise filter 382, and the other end is connected to one end of each halogen heater 331,332. The cathode of the regenerative diode D1 is also connected to one end of the reactor L1. A switching element SW is connected between the anode of the regenerative diode D1 and the second output terminal 1B of the rectifier 381. The switching element SW is, for example, an IGBT, the collector is connected to the anode of the regenerative diode D1, and the emitter is connected to the second output terminal 1B of the rectifying unit 381. The switching element SW is turned on / off according to the level of the pulse signal VP applied to the gate by the IGBT drive circuit 384. That is, the emitter-collector is made conductive or cut off.

  The IGBT drive circuit 384 receives the notification signal NTF indicating the duty ratio from the PWM control unit 30C, and applies a pulse signal VP having a PWM frequency (for example, 20 kHz) to the gate of the switching element SW of the step-down chopper 383 at the duty ratio. To do. In response to this, the switching element SW is turned on / off at the same duty ratio as that of the pulse signal VP.

  The switch unit 385 is an electromagnetic relay including, for example, four pairs of double switches S1, S2, S3, and S4. Each of the switches S1,... Is turned on / off in response to an open / close signal SSW from the PWM control unit 30C. The first switch S1 opens and closes simultaneously between the output terminal of the reactor L2 of the step-down chopper 383 and one end of the first halogen heater 331, and between the anode of the regenerative diode D1 and the other end of the first halogen heater 331. The second switch S1 opens and closes simultaneously between the output terminal of the reactor L2 of the step-down chopper 383 and one end of the second halogen heater 332, and between the anode of the regenerative diode D1 and the other end of the second halogen heater 332. With these two pairs of double switches S1 and S2, the switch unit 385 switches the output destination of the DC voltage VO by the step-down chopper 383 to the application target selected by the PWM control unit 30C. The third switch S3 simultaneously opens and closes between the output terminals 1A and 1B of the rectifying unit 381 and both ends of the first halogen heater 331, and the fourth switch S4 includes the output terminals 1A and 1B of the rectifying unit 381 and the second halogen heater 332. Open and close between the two ends at the same time. The switch unit 385 turns off the first switch S1 when turning on the third switch S3, and turns off the second switch S2 when turning on the fourth switch S4. Thus, the third switch S3 and the fourth switch S4 form a conduction path that bypasses the step-down chopper 383 between the rectifying unit 381 and the halogen heaters 331 and 332. The output voltage VR of the rectifying unit 381 is directly applied to the halogen heaters 331 and 332 through this path.

-Current / voltage waveform-
FIG. 6 is a graph showing a current / voltage waveform in each part of the switching converter 30S. (A) shows the waveform of the input voltage VI of the rectifying unit 381, (b) shows the waveform of the output voltage VR of the rectifying unit 381, and (c) shows that the IGBT drive circuit 384 is connected to the gate of the switching element SW. The waveform of the pulse signal VP to be applied is shown. (D) shows the waveform of the voltage VD across the regenerative diode D1 when the step-down chopper 383 is connected to at least one of the halogen heaters 331 and 332, and (e) shows the output of the step-down chopper 383 in the same case. The waveform of current IO is shown. In each graph, the horizontal axis represents time.

  As shown in FIGS. 6A and 6B, the waveform of the output voltage VR of the rectifying unit 381 is equal to the shape obtained by vertically inverting the negative portion of the waveform of the input voltage VI. As shown in FIG. 6C, the pulse signal VP is a rectangular binary signal, and the high level HL represents the ON period of the switching element SW, and the low level LL represents the OFF period. As shown in FIGS. 6C, 6D, and 6E, when the pulse signal VP is at the high level HL, the voltage VD across the regeneration diode D1 is substantially equal to the output voltage VR of the rectifier 381. And the output current IO of the step-down chopper 383 increases. On the other hand, when the pulse signal VP is at the low level LL, the voltage VD across the regenerative diode D1 is substantially equal to “0”, and the output current IO of the step-down chopper 383 is substantially interrupted. These are due to the following reasons.

  During the ON period of the switching element SW, the anode of the regenerative diode D1 is conducted to the second output terminal 1B of the rectifying unit 381. On the other hand, since the cathode is connected to the first output terminal 1A of the rectifying unit 381, the regenerative diode D1 is turned off by the reverse bias. Thus, the voltage VD across the regenerative diode D1 substantially coincides with the voltage VR between the output terminals 1A and 1B of the rectifier 381. Therefore, the current IO flowing in the series connection between the reactor L2 of the step-down chopper 383 and the halogen heaters 331 and 332 increases, and the electric power from the external power source OPS is accumulated as magnetic energy in the reactor L2, and the halogen heaters 331 and 332 Is released as radiant heat.

  During the OFF period of the switching element SW, the anode of the regenerative diode D1 is electrically separated from the second output terminal 1B of the rectifying unit 381. On the other hand, in reactor L2 of step-down chopper 383, an electromotive force is generated by the magnetic energy accumulated during the ON period, and output current IO is maintained. Since the output current IO flows into the anode of the regenerative diode D1, the regenerative diode D1 is turned on. As a result, the voltage VD across the regenerative diode D1 drops substantially to “0”.

  Further, the inductance of the reactor L2 and the on / off cycle of the switching element SW, that is, the reciprocal of the PWM frequency (PWM cycle) are designed so that the reactor L2 uses up magnetic energy during the off period and the output current IO is interrupted. As described above, the operation mode of the step-down chopper 383 in which the output current IO is interrupted every time the switching element SW is turned off is referred to as “current discontinuous mode”. Thus, the output current IO is interrupted during the OFF period of the switching element SW. Therefore, the heat radiation from the halogen heaters 331 and 332 stops.

  During the ON period of the switching element SW, the magnetic flux increases in the reactor L2 of the step-down chopper 383 as the magnetic energy is accumulated. The increase Δφon at this time is equal to the difference VR−VO between the input voltage VR from the rectifying unit 381 and the output voltage VO to the halogen heaters 331 and 332 during the ON period. It is approximated by a value multiplied by the SW on-time Ton: Δφon≈ (VR−VO) Ton. On the other hand, in the OFF period of the switching element SW, magnetic energy is released from the reactor L2 of the step-down chopper 383, so that the magnetic flux decreases. If the change during the OFF period of the output voltage VO is ignored, the decrease Δφoff at this time is equal to a value obtained by multiplying the output voltage VO by the OFF time Toff of the switching element SW: Δφoff≈VO × Toff. If the operation of the step-down chopper 383 is stable, the increase Δφon and the decrease Δφoff of the magnetic flux should be equal. From the equation Δφon = Δφoff, that is, (VR−VO) / VO = Toff / Ton, the ratio of the output voltage VO to the input voltage VR is expressed by the following equation by the duty ratio DT = Ton / (Ton + Toff) of the switching element SW Expressed: VO / VR = DT. In particular, since the duty ratio DT is smaller than “1”, the output voltage VO is lower than the input voltage VR: VO <VR.

-Advantages of current discontinuous mode-
The step-down chopper 383 is designed to operate in the current discontinuous mode as described above. As a result, the output current IO is maintained during the ON period of the switching element SW, so that heat is radiated from the halogen heaters 331 and 332, and the output current IO is interrupted during the OFF period, so that the heat emission from the halogen heaters 331 and 332 stops. The higher the ratio of the time length of the on period to the off period, that is, the duty ratio of on / off of the switching element SW, the greater the amount of heat generated by the halogen heaters 331 and 332. Therefore, the amount of heat generated by the halogen heaters 331 and 332 is controlled by adjusting the duty ratio.

  Depending on the inductance of the reactor L2 and the PWM frequency, as shown in FIG. 6E by a broken line, an operation of maintaining the output current IO during the OFF period of the switching element SW is also possible. This operation mode of the step-down chopper 383 is referred to as “current continuous mode”. Even in the current continuous mode, the output current IO is attenuated during the OFF period of the switching element SW. Therefore, the amount of heat generated by the halogen heaters 331 and 332 can be adjusted by adjusting the ON / OFF duty ratio of the switching element SW.

However, in the current discontinuous mode, unlike the current continuous mode, the output current IO is interrupted during the off period, so that there is substantially no switching noise associated with the switching element SW being turned on. This significantly reduces switching noise, so that the current discontinuous mode is more advantageous than the current continuous mode.
“Switching noise” mainly refers to a surge current / voltage associated with ON / OFF of the switching element SW. If the switching noise is sufficiently small, it is removed by the noise filter 382. However, the switching noise is so large that it cannot be removed by the noise filter 382. If the switching noise propagates to the external power source OPS, it may cause a malfunction in the external device through the power source OPS. Excessive switching noise can also enter the power supply system of other elements of the printer 100 such as the transport unit 10 and the image forming unit 20 through the connection part between the switching conversion unit 30S and the external power supply OPS. In this case, the operation at the intrusion destination becomes unstable, and there is a risk that problems such as poor conveyance and image quality degradation may occur.

  FIG. 6F is a graph showing waveforms of the pulse signal VP, the emitter current IE, and the collector-emitter voltage VCE before and after the switching element SW is turned on. In the current discontinuous mode, the output current IO has already stopped before the pulse signal VP rises. Therefore, when the pulse signal VP rises, no current flows into the collector of the switching element SW, so that the emitter current IE does not flow out. Only after the collector-emitter voltage VCE drops to substantially "0", the emitter current IE starts to flow. That is, as indicated by the solid line in FIG. 6F, the period in which the emitter current IE flows does not overlap with the period in which the collector-emitter voltage VCE is greater than “0”. On the other hand, in the current continuous mode, the output current IO maintains a certain amount during the OFF period of the switching element SW. Accordingly, a part of the output current IO flows into the collector of the switching element SW in response to the rise of the pulse signal VP, so that the emitter current IE rapidly increases before the collector-emitter voltage VCE substantially drops to “0”. Increase. That is, as shown in FIG. 6F by a broken line, the period during which the emitter current IE flows overlaps with the period when the collector-emitter voltage VCE is greater than “0”. In this overlap period SWL, the product of the emitter current IE and the collector-emitter voltage VCE is not “0”, so that power loss occurs in the switching element SW. Part of this loss is dissipated as switching noise. In this way, in the current discontinuous mode, there is no waveform overlap between the emitter current IE and the collector-emitter voltage VCE, so that there is substantially no switching noise associated with the switching element SW being turned on.

[Control pattern for halogen heater]
As described above, the amount of heat generated by the halogen heaters 331 and 332 is controlled by adjusting the ON / OFF duty ratio of the switching element SW. Therefore, the usable range of the heat generation amount of the halogen heaters 331 and 332 is determined by the range in which the duty ratio of the on / off of the switching element SW can be set.

  FIG. 7A is a graph showing the usable output power ranges of the first halogen heater 331 and the second halogen heater 332. The output power of each heater 331, 332 is equal to the rated power of the heater during continuous lighting. The continuous lighting is theoretically equivalent to a case where the duty ratio of on / off of the switching element SW is 100% (that is, “all lighting”), so the rated power of the heater represents the maximum value of the output power. In FIG. 7A, the maximum value of the output power of the first halogen heater 331 is 1000 W, and the maximum value of the second halogen heater 332 is 500 W. The rated power of each of the heaters 331 and 332 is 1000 W, Because it is 500W.

When the switching converter 30S blinks the heaters 331 and 332 by PWM control, as shown in FIG. 7A, the upper limit of the range that can be set for the on / off duty ratio of the switching element SW is more than 100%. The lower limit is higher than 0%.
The upper limit of the duty ratio is determined by the condition that the step-down chopper 383 can operate in the current discontinuous mode, and is particularly greatly influenced by the inductance of the reactor L2 of the step-down chopper 383 and the PWM cycle. The upper limit 70% of the duty ratio shown in FIG. 7A is an example of an allowable upper limit calculated from the inductance 20 μH of the reactor L2 and the PWM frequency 20 kHz. In particular, for the purpose of simplifying the control, both heaters 331 and 332 are used. It is common to.

  The lower limit of the duty ratio is whether or not the heat generation amount of the halogen heaters 331 and 332 can maintain the halogen cycle, that is, whether or not the heat from the heater can maintain the tube wall temperature at the lower limit of 250 ° C. or more. Determined by. The lower limit 30% of the duty ratio shown in FIG. 7A is an example of an allowable lower limit, and is commonly used for both heaters 331 and 332 for the purpose of simplifying the control.

  FIG. 7B is a table showing control patterns TY1, TY2,..., TY6 for the halogen heaters 331 and 332 by the switching converter 30S. The control pattern TY1,... Defines whether the state of each of the halogen heaters 331, 332 should be maintained as full lighting, blinking by PWM control, or extinguishing, specifically, four pairs including the switch unit 385. Are expressed by a combination of ON and OFF of the two-unit switches S1,. The range of output power in each control pattern TY1,... Is shown in FIG.

  In the first control pattern TY1, the switch unit 385 turns off the first switch S1 and the second switch S2, and turns on the third switch S3 and the fourth switch S4. As a result, the output voltage VR of the rectifying unit 381 bypasses the step-down chopper 383 and is directly applied to both the halogen heaters 331 and 332, so that all the halogen heaters 331 and 332 are fully lit. The output power 1500W in this case is equal to the sum of the rated powers of both heaters 331 and 332, 1000W + 500W = 1500W.

  In the second control pattern TY2, the switch unit 385 turns off the first switch S1 and the fourth switch S4, and turns on the second switch S2 and the third switch S3. As a result, the first halogen heater 331 is fully lit, while the second halogen heater 332 blinks by intermittent output according to the PWM control from the step-down chopper 383. The output power range 1150W to 1350W in this case is higher than the output power range 150W to 350W of the second halogen heater 332 by the rated power 1000W of the first halogen heater 331.

  In the third control pattern TY3, the switch unit 385 turns on the first switch S1 and the fourth switch S4 and turns off the second switch S2 and the third switch S3. Accordingly, the first halogen heater 331 blinks by intermittent output according to the PWM control from the step-down chopper 383, while the second halogen heater 332 is fully lit. The output power range 800 W to 1200 W in this case is higher than the output power range 300 W to 700 W of the first halogen heater 331 by the rated power 500 W of the second halogen heater 332.

  In the fourth control pattern TY4, the switch unit 385 turns on the first switch S1 and the second switch S2, and turns off the third switch S3 and the fourth switch S4. As a result, both halogen heaters 331 and 332 blink by intermittent output according to the PWM control from the step-down chopper 383. The output power range 450W to 1050W in this case is equal to the range from the lower limit sum 300W + 150W to the upper limit sum 700W + 350W between the output powers of the heaters 331 and 332.

  In the fifth control pattern TY5, the switch unit 385 turns on the first switch S1, and turns off the remaining switches S2, S3, and S4. Thereby, the first halogen heater 331 blinks by intermittent output according to the PWM control from the step-down chopper 383, while the second halogen heater 332 is turned off. The output power range 300W to 700W in this case is equal to the output power range of the first halogen heater 331.

  In the sixth control pattern TY6, the switch unit 385 turns on the second switch S2, and turns off the remaining switches S1, S3, and S4. As a result, the first halogen heater 331 is turned off, while the second halogen heater 332 blinks by intermittent output according to the PWM control from the step-down chopper 383. The output power range 150 W to 350 W in this case is equal to the output power range of the second halogen heater 331.

  Since the temperature rise control is performed during the warm-up period of the printer 100, the heat generation amount required for the halogen heaters 331 and 332 is the maximum at least in the initial stage of the temperature rise control. Temperature control is performed during the printing period and the standby period. In this control, it is only necessary to compensate for the amount of heat escaping from the heating roller 31, and therefore the amount of heat generation required for the halogen heaters 331 and 332 is smaller than that in the temperature increase control. However, the amount of heat required for the halogen heaters 331 and 332 is larger in the printing period than in the standby period because the amount of heat taken from the heating roller 31 to the sheet SH2 during the fixing process needs to be compensated. Therefore, for example, as shown in FIG. 7A, the PWM control unit 30 </ b> C is configured to warm the printer 100 from among the control patterns TY1, TY2, TY3, and TY4 that have an output power range (even at least) belonging to 1000 W or more. The pattern used for the up period is selected, the pattern used for the printing period of the printer 100 is selected from the control patterns TY3, TY4, and TY5 belonging to the output power range of 500 W or more and less than 1000 W, and the output power range is 500 W. A pattern to be used during the standby period of the printer 100 is selected from the control patterns TY4, TY5, and TY6 belonging to less than.

[Temperature control of heating roller]
FIG. 8A is a graph showing a change with time of the temperature of the heating roller 31 after the startup time t0 of the printer 100. In response to activation of the printer 100, the fixing unit 30 starts temperature control of the heating roller 31. Specifically, during the warm-up period WUP of the printer 100, the PWM control unit 30C performs temperature increase control. The PWM control unit 30C first sets the target temperature of the heating roller 31 to a value Ttg during printing, for example, 180 ° C. Next, the PWM control unit 30C selects a control pattern for the halogen heaters 331 and 332 from the difference between the target temperature Ttg and the initial temperature T0 of the heating roller 31 indicated by the output of the temperature sensor 35 at the start time t0.

  FIG. 8B is a correspondence table between the initial temperature T0 of the heating roller 31 and the control pattern selected by the PWM control unit 30C according to the temperature. For example, when the initial temperature T0 is less than 30 ° C., the difference from the target temperature Ttg is wide, for example, 150 degrees or more, so the first control pattern TY1 with the maximum output power is selected. The PWM control unit 30C selects a control pattern in which the output power can be set lower as the initial temperature T0 and the actual temperature of the heating roller 31 are closer to the target temperature Ttg. In the example shown in FIG. 8B, when the initial temperature T0 is 30 ° C. to 100 ° C., the difference from the target temperature Ttg is narrowed to 80 ° C. to 150 ° C., so the second control pattern with the next largest output power. When TY2 is selected and the initial temperature T0 is 100 ° C. to 150 ° C., the difference from the target temperature Ttg is further narrowed to 30 ° to 80 ° C., so the third control pattern TY3 is selected, and the initial temperature is 150 ° C. to 180 ° C. When ° C. = Ttg, the difference from the target temperature Ttg shrinks to 0 to 30 degrees, so the fourth control pattern TY4 is selected. As the actual temperature of the heating roller 31 is closer to the target temperature Ttg by such selection of the control pattern, the rate of temperature rise is suppressed to be lower. Therefore, the actual temperature of the heating roller 31 rises even after reaching the target temperature Ttg. Subsequently, a phenomenon that greatly exceeds the target temperature Ttg, that is, overshoot is prevented. Thus, the warm-up period WUP is sufficiently shortened.

  In the example shown in FIG. 8A, the printer 100 starts printing immediately from the end time t1 of the warm-up period WUP. In the printing period PRT, each time the sheet SH2 is passed through the nip between the heating roller 31 and the pressure roller 32, a large amount of heat is taken from the heating roller 31 to the sheet SH2, so that the temperature of the heating roller 31 increases. fluctuate. In order to prevent this fluctuation, that is, excessive temperature ripple RPL, it is necessary to quickly compensate for the amount of heat lost by the heating roller 31. The PWM control unit 30C selects one of the third control pattern TY3, the fourth control pattern TY4, and the fifth control pattern TY5 so that this amount of heat can be sufficiently compensated.

  At the end time t2 of the job processing, the printer 100 shifts from the operation mode to the standby mode. Accordingly, the PWM control unit 30C sets the target temperature to a value Twt (for example, 150 ° C.) lower than the value Ttg in the print period PRT. Change to Due to this change, the difference between the actual temperature of the heating roller 31 and the target temperature Twt is relatively large immediately after the start of the standby period WTT, so the PWM control unit 30C first selects the fifth control pattern TY5. That is, only the first halogen heater 331 blinks by PWM control, and the second halogen heater 332 is turned off. Since the PWM control for the first halogen heater 331 has a heat quantity that can be set larger than that for the second halogen heater 332, it is easy to suppress the temperature drop rate of the heating roller 31 to an appropriate level without increasing it.

  As the difference between the temperature of the heating roller 31 and the target temperature Twt narrows, the amount of heat necessary to suppress the temperature fluctuation of the heating roller 31 decreases, so the duty ratio decreases and eventually falls below the allowable lower limit of 30%. When the state where the duty ratio is less than the allowable lower limit 30% is maintained for a predetermined time, for example, 10 seconds, the PWM control unit 30C changes the fifth control pattern TY5 to the fifth control pattern TY5 as indicated by an arrow TR1 in FIG. Switch to 6 control pattern TY6. That is, the first halogen heater 331 is turned off, and only the second halogen heater 332 blinks by PWM control. The PWM control for the second halogen heater 332 has a smaller amount of heat that can be set than that for the first halogen heater 331, so that the temperature is stably stabilized at the target value Twt without giving excessive ripple to the temperature of the heating roller 31. Make it. Furthermore, since the duty ratio is maintained at the lower limit of 30% or more by switching the control pattern, the first halogen heater 331 is prevented from continuing to be lit when the tube wall temperature is below the lower limit, and its long life Can be maintained.

  In the standby period WTT, the difference between the temperature of the heating roller 31 and the target temperature Twt may be greatly increased due to, for example, a large change in the environmental temperature of the printer 100 due to replacement of outside air. In this case, since the amount of heat to be given to the heating roller 31 increases, the duty ratio increases, and there is a possibility that the upper limit of 70% is exceeded. When the state in which the duty ratio exceeds the upper limit of 70% is maintained for a predetermined time, for example, 10 seconds, the PWM control unit 30C sets the sixth control pattern TY6 to the fifth value as illustrated in FIG. Switch to the control pattern TY5. That is, only the first halogen heater 331 blinks by PWM control, and the second halogen heater 332 is turned off. As a result, the amount of heat that can be supplied to the heating roller 31 increases, so that it is easy to quickly return the temperature of the heating roller 31 to the target temperature Twt. Furthermore, since the duty ratio is maintained at the upper limit of 70% or less by switching the control pattern, the step-down chopper 383 is maintained in the current discontinuous mode, and the bad influence caused by the switching noise is caused by the external power supply OPS or other of the printer 100. It can be prevented from reaching the element.

  At time t3 when the printer 100 receives a new job processing request, the printer 100 shifts from the standby mode to the operation mode. In response to this, the PWM control unit 30C changes the target temperature of the heating roller 31 from the value Twt in the standby period WTT to the value Ttg during printing. Due to this change, immediately after the start time t3 of the recovery period RCV, the difference between the actual temperature of the heating roller 31 and the target temperature Ttg opens relatively large, so that the duty ratio increases. If, for example, 10 seconds elapses after the duty ratio exceeds the upper limit of 70%, the PWM control unit 30C switches the sixth control pattern TY6 to the fifth control pattern TY5. Even at the time of switching, if the difference between the actual temperature of the heating roller 31 and the target temperature Ttg is sufficiently large, the duty ratio continues to increase. When the duty ratio reaches the upper limit of 70% again and further exceeds that, for example, if the state continues for 10 seconds, the PWM control unit 30C determines that the fifth state as shown by the arrow TR3 in FIG. The control pattern TY5 is switched to the fourth control pattern TY4. That is, in addition to the first halogen heater 331, the second halogen heater 332 blinks by PWM control. Thus, since the heat quantity which can be set increases sequentially, it is easy to prevent overshooting by suppressing the heating rate of the heating roller 31 to an appropriate level without making it excessive. Furthermore, since the duty ratio is maintained at the upper limit of 70% or less by switching the control pattern, the step-down chopper 383 is maintained in the current discontinuous mode, and the bad influence caused by the switching noise is caused by the external power supply OPS or other of the printer 100. It can be prevented from reaching the element.

-Flow chart of temperature control-
FIG. 9 is a flowchart of temperature control of the heating roller 31 by the fixing unit 30. This control is started when the fixing unit 30 receives an instruction to shift to the operation mode from the main control unit 60.
In step S101, the printer 100 performs warm-up or recovery as the printer 100 shifts to the operation mode. During this period WUP, RCV, the PWM control unit 30C performs temperature rise control. Specifically, the PWM control unit 30C first sets the target temperature of the heating roller 31 to the value Ttg at the time of printing, and between the target temperature Ttg and the initial temperature T0 of the heating roller 31 indicated by the output of the temperature sensor 35. A control pattern for the halogen heaters 331 and 332 is selected from the difference. Next, the PWM control unit 30C selects a pulse voltage application target according to the selected control pattern. That is, the on / off combination of the four pairs of double switches S1,. The PWM control unit 30C instructs this combination to the switch unit 385 with the open / close signal SSW. When the first control pattern TY1 is selected, the PWM control unit 30C causes the switch unit 385 to turn off the first switch S1 and the second switch S2, and to turn on the third switch S3 and the fourth switch S4. All the halogen heaters 331 and 332 are also lit. When a pattern other than the first control pattern TY1 is selected, the PWM control unit 30C subsequently calculates the duty ratio of the pulse voltage based on the difference between the temperature of the heating roller 31 indicated by the output of the temperature sensor 35 and the target temperature Ttg. Then, the calculated value is instructed to the switching conversion unit 30S by the notification signal NTF. Thereafter, the PWM control unit 30C monitors the difference between the temperature of the heating roller 31 and the target temperature Ttg through the temperature sensor 35, updates the duty ratio according to the difference, and notifies the updated duty ratio to the notification signal NTF. To instruct the switching converter 30S. Thereafter, the process proceeds to step S102.

In step S102, the PWM control unit 30C confirms whether or not the temperature of the heating roller 31 has reached the target temperature Ttg. If so, the process proceeds to step S103. If not, the process repeats step S101.
In step S103, since the temperature of the heating roller 31 has reached the target temperature Ttg, the PWM control unit 30C changes the temperature increase control to the temperature control. At this time, the PWM control unit 30C confirms the operation mode of the printer 100, and selects a target temperature of the heating roller 31 and a control pattern for the halogen heaters 331 and 332 according to the operation mode. For example, since printing is started in the operation mode, a large amount of heat is deprived from the heating roller 31 to the sheet SH2 every time the sheet SH2 is passed through the nip between the heating roller 31 and the pressure roller 32. Therefore, a control pattern that generates a large amount of heat so that the deprived heat amount can be compensated in the heating roller 31, for example, the third control pattern TY3, the fourth control pattern TY4, or the fifth control pattern TY5 is selected. The On the other hand, in the standby mode, it is difficult for heat to escape from the heating roller 31, so that the amount of heat to be compensated is relatively small. Therefore, a control pattern that generates a relatively small amount of heat, for example, the fifth control pattern TY5 or the sixth control pattern TY6 is selected. In the temperature control, similarly to the temperature increase control, the PWM control unit 30C monitors the difference between the temperature of the heating roller 31 and the target temperature Ttg through the temperature sensor 35, updates the duty ratio according to the difference, and updates it. The subsequent duty ratio is instructed to the switching converter 30S by the notification signal NTF. Thereafter, the process proceeds to step S104.

In step S104, the PWM control unit 30C confirms whether or not the main control unit 60 has instructed to stop the fixing unit 30 such as shifting to the sleep mode or turning off the printer 100. If so, the process proceeds to step S105, and if not, the process repeats step S103.
In step S105, since the main control unit 60 instructs the stop of the fixing unit 30, the PWM control unit 30C causes the switch unit 385 to turn off all the switches S1, ..., S4 and turn off the halogen heaters 331 and 332. . Thereafter, the process ends.

-Flow chart of temperature control in standby mode-
FIG. 10 is a flowchart of the control in the standby mode in the temperature control in step S103 shown in FIG.
In step S201, the PWM control unit 30C confirms whether the printer 100 is in the standby mode. If it is the standby mode, the process proceeds to step S202, and if it is another mode, the process returns to the main routine shown in FIG.

In step S202, since the printer 100 is in the standby mode, the PWM control unit 30C first performs temperature control with the fifth control pattern TY5. That is, only the first halogen heater 331 is blinked by PWM control, and the second halogen heater 332 is turned off. Thereafter, the process proceeds to step S203.
In step S203, the PWM control unit 30C confirms whether or not the changed duty ratio falls below the allowable lower limit of 30%. If so, the process proceeds to step S204, and if above, the process repeats step S201.

In step S204, since the changed duty ratio is below the allowable lower limit of 30%, the PWM control unit 30C checks whether or not the duration of the state has reached the threshold of 10 seconds or more. If it has reached the threshold value or more, the process proceeds to step S205, and if it is less, the process repeats step S201.
In step S205, since the duration of the state in which the changed duty ratio is below the allowable lower limit 30% has reached the threshold value or more, the PWM control unit 30C confirms whether the printer 100 is in the standby mode. If it is the standby mode, the process proceeds to step S206, and if it is any other mode, the process returns to the main routine shown in FIG.

  In step S206, the temperature control is continued because the printer 100 is in the standby mode. However, since the duty ratio falls below the allowable lower limit in the fifth control pattern TY5, the PWM control unit 30C switches the fifth control pattern TY5 to the sixth control pattern TY6 with a smaller amount of heat generation. That is, instead of the first halogen heater 331, only the second halogen heater 332 blinks by PWM control, and the first halogen heater 331 is turned off. Thereafter, the process proceeds to step S207.

In step S207, the PWM control unit 30C confirms whether or not the changed duty ratio exceeds the allowable upper limit of 70%. If so, the process proceeds to step S208, and if below, the process repeats step S205.
In step S208, since the changed duty ratio exceeds the allowable upper limit of 70%, the PWM control unit 30C checks whether or not the duration of the state has reached the threshold value of 10 seconds or more. If it reaches the threshold value or more, the process repeats step S201, and if it is less, the process repeats step S205.

In step S201 following step S208, the duration of the state in which the changed duty ratio exceeds the allowable upper limit of 70% has reached the threshold value or more, so the PWM control unit 30C determines whether or not the printer 100 is in the standby mode. Check. If it is the standby mode, the process proceeds to step S202, and if it is another mode, the process returns to the main routine shown in FIG.
In step S202 following step S208, the temperature control is continued because the printer 100 is in the standby mode. However, since the duty ratio exceeds the allowable upper limit in the sixth control pattern TY6, the PWM control unit 30C switches the sixth control pattern TY6 to the fifth control pattern TY5 with a larger amount of heat generation. That is, instead of the second halogen heater 332, only the first halogen heater 331 is blinked by PWM control, and the second halogen heater 332 is turned off. Thereafter, the process proceeds to step S203.

-Flow chart of PWM control-
FIG. 11 is a flowchart of PWM control for the halogen heaters 331 and 332. This control is started when the PWM control unit 30C selects a control pattern other than the first control pattern TY1, and is repeated until the control pattern is switched to another control pattern.
In step S301, the temperature sensor 35 measures the temperature of the heating roller 31 from the output of the built-in thermopile, and feeds back the measured value to the PWM control unit 30C. Thereafter, the process proceeds to step S302.

In step S302, the PWM control unit 30C compares the measured value of the temperature of the heating roller 31 fed back from the temperature sensor 35 with the target temperature, and calculates the duty ratio of the pulse voltage based on the difference between the two temperatures. Thereafter, the process proceeds to step S303.
In step S303, the PWM control unit 30C notifies the calculated value in step S302 to the IGBT drive circuit 384 of the switching conversion unit 30S with the notification signal NTF. In response to the notification signal NTF, the IGBT drive circuit 384 decodes the duty ratio from the notification signal NTF and applies the pulse signal VP to the gate of the switching element SW based on the ratio. Thereafter, the processing returns to the main routine shown in FIG.

[Advantages of the embodiment]
In the printer 100 according to the embodiment of the present invention, as described above, the fixing unit 30 controls the temperature of the heating roller 31 using the two halogen heaters 331 and 332 having different rated powers. In particular, in the temperature control in the standby mode, the PWM control unit 30C first selects the fifth control pattern TY5, blinks only the first halogen heater 331 having a high rated power by PWM control, and the second halogen heater having a low rated power. 332 is turned off. At this time, the allowable lower limit of 30% of the duty ratio of the pulse voltage is determined by experiment or simulation so as to satisfy the condition that the heat generation amount of the first halogen heater 331 can maintain the halogen cycle, and is incorporated in the PWM controller 30C. Stored in a memory device. As the difference between the temperature of the heating roller 31 and the target temperature Twt decreases, the duty ratio calculated by the PWM control unit 30C decreases. When the duty ratio falls below the allowable lower limit of 30%, the PWM control unit 30C switches the pulse voltage application target from the first halogen heater 331 to the second halogen heater 332. Since the second halogen heater 332 has a lower rated power than the first halogen heater 331, the duty ratio of the corresponding pulse voltage is high even if the necessary heat generation amount is common. Therefore, PWM control can be continued while maintaining the duty ratio at or above the allowable lower limit. In this way, the printer 100 continues PWM control without causing excessive noise to the external power supply system while maintaining the halogen cycle in the halogen heaters 331 and 332, and further reduces the amount of heat applied to the heating roller 31. Can be stabilized.

  During the temperature control in the standby mode, when the difference between the temperature of the heating roller 31 and the target temperature Twt is greatly opened and the duty ratio of the pulse voltage exceeds the allowable upper limit 70%, the PWM control unit 30C applies the pulse voltage. The target is switched from the second halogen heater 332 to the first halogen heater 331. Since the first halogen heater 331 has a higher rated power than the second halogen heater 332, the duty ratio of the pulse voltage corresponding to the same calorific value is low. Therefore, it is easy to continue the PWM control while maintaining the duty ratio below the allowable upper limit, and to quickly return the temperature of the heating roller 31 to the target temperature Twt. In particular, since the step-down chopper 383 maintains the current discontinuous mode, an adverse effect caused by the switching noise does not affect the external power supply OPS or other elements of the printer 100.

[Modification]
(A) The image forming apparatus 100 according to the above embodiment is a color laser printer. In addition, the image forming apparatus according to the embodiment of the present invention may be any one that thermally fixes a toner image on a sheet, such as a monochrome laser printer, a facsimile, a copier, or a multifunction peripheral (MFP).

(B) The switching converter 30 </ b> S shown in FIG. 5 includes a step-down chopper 383. In addition, the switching converter 30S may include other non-insulated switching converters such as a step-up chopper and a step-up / step-down chopper. The switching element SW shown in FIG. 5 is an IGBT, but other types of power transistors such as FETs may be used instead.
(C) In FIG. 7A, the allowable upper limit and the allowable lower limit of the duty ratio are common between the two halogen heaters 331 and 332. This is only a setting for the purpose of simplifying temperature control such as simplification of the structure of the switch unit 385. Actually, the duty ratio that satisfies the condition that the calorific value of the halogen heater can maintain the halogen cycle, that is, the condition that the heat from the heater can maintain the tube wall temperature at the lower limit of 250 ° C. or higher is the glass ratio. It depends on the thermal characteristics of the halogen heater, such as the heat capacity of the tube. Similarly, the duty ratio that satisfies the condition that the step-down chopper 383 can operate in the current discontinuous mode varies depending on the electrical characteristics of the halogen heater, such as resistance. Therefore, for example, the allowable upper limit or the allowable lower limit of the duty ratio may be different for each halogen heater by executing PWM control for different halogen heaters with different step-down choppers.

  The present invention relates to temperature control of a fixing unit in an electrophotographic image forming apparatus. As described above, the PWM control unit according to the present invention changes the lower limit of the duty ratio of the pulse voltage that satisfies the condition that the heat generation amount of the two halogen heaters with the higher rated power can maintain the halogen cycle. When the duty ratio falls below, the pulse voltage application target is switched to a halogen heater with a low rated power. Thus, the present invention is clearly industrially applicable.

DESCRIPTION OF SYMBOLS 100 Printer 10 Feed part 20 Image forming part 30 Fixing part 40 Paper discharge part 31 Heating roller 32 Pressure roller 331 First halogen heater 332 Second halogen heater 35 Temperature sensor 30S Switching conversion part 381 Rectifier 382 Noise filter 383 Step-down chopper SW switching element 384 IGBT drive circuit VP pulse signal 385 Switch unit S1, S2, S3, S4 Double switch 30C PWM control unit SSW Open / close signal NTF Notification signal

Claims (8)

An image forming unit for forming a toner image on a sheet;
A fixing unit for thermally fixing the toner image to the sheet;
An image forming apparatus comprising:
The fixing unit is
A heat transfer member that contacts the sheet and transfers heat;
A first halogen heater for heating the heat transfer member;
A second halogen heater having a lower rated power than the first halogen heater and heating the heat transfer member;
The actual temperature of the heat transfer member is monitored, and one or both of the first halogen heater and the second halogen heater is selected as the pulse voltage application target in accordance with the difference between the actual temperature and the target temperature. A pulse width modulation (PWM) control unit that changes the duty ratio of the pulse voltage over time according to the change with time of the difference,
A switching converter that applies a pulse voltage to a pulse voltage application target selected by the PWM controller at a duty ratio changed by the PWM controller;
Have
In the case where the PWM control unit has selected the first halogen heater as a pulse voltage application target,
Check whether the duty ratio after the change is lower than the lower limit of the duty ratio of the pulse voltage satisfying the condition that the heat generation amount of the first halogen heater can maintain the halogen cycle,
When the duty ratio after the change is less than the lower limit, the application target of the pulse voltage is switched from the first halogen heater to the second halogen heater.   2. The image according to claim 1, wherein the PWM control unit checks whether the changed duty ratio falls below the lower limit during a standby period between the image forming unit and the fixing unit. Forming equipment. In the case where the PWM control unit has selected the first halogen heater as a pulse voltage application target,
Check whether the duty ratio after the change exceeds the first upper limit of the duty ratio of the pulse voltage that satisfies the condition that the switching converter is operable in the current discontinuous mode,
3. The image forming apparatus according to claim 1, wherein when the changed duty ratio exceeds the first upper limit, the second halogen heater is added as a pulse voltage application target. 4.   The PWM control unit confirms whether the changed duty ratio exceeds the first upper limit, at least one of a warm-up period, a recovery period, and a print period between the image forming unit and the fixing unit The image forming apparatus according to claim 3, wherein the image forming apparatus is performed during the period. In the case where the PWM control unit has selected the second halogen heater as a pulse voltage application target,
Check whether the duty ratio after the change exceeds the second upper limit of the duty ratio of the pulse voltage that satisfies the condition that the switching converter can operate in the current discontinuous mode,
5. The pulse voltage application target is switched from the second halogen heater to the first halogen heater when the duty ratio after the change exceeds the second upper limit. The image forming apparatus described.   The PWM control unit checks whether the duty ratio after the change exceeds the second upper limit, at least one of a warm-up period, a recovery period, and a print period between the image forming unit and the fixing unit The image forming apparatus according to claim 5, which is performed during the period. The switching converter is
A drive circuit for generating a pulse signal at a duty ratio changed by the PWM control unit;
A step-down chopper that includes a switching element that turns on and off according to the pulse signal, and that outputs a pulse voltage at a duty ratio changed by the PWM control unit;
A switch unit for switching a pulse voltage output destination from the step-down chopper to a pulse voltage application target selected by the PWM control unit;
Have
The PWM control unit causes the switch unit to switch the output destination of the pulse voltage from the step-down chopper while the switching element of the step-down chopper is turned off. The image forming apparatus according to any one of the above. The switch part is
An electromagnetic relay that opens and closes a contact between the step-down chopper and the first halogen heater and opens and closes a contact between the step-down chopper and the second halogen heater according to a signal from the PWM control unit The image forming apparatus according to claim 7.

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